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BUREAU OF MINES 
INFORMATION CIRCULAR/1990 

D<e7/ 

Z70 




Multislice Mining for Thick 
Western Coal Seams 



By T. D. Hackett, D. L. Boreck, 
and D. R. Clarke 




^0 OF ^ 



U.S. BUREAU OF MINES 
1910-1990 



THE MINERALS SOURCE 



Mission: As the Nation's principal conservation 
agency, the Department of the Interior has respon- 
sibility for most of our nationally-owned public 
lands and natural and cultural resources. This 
includes fostering wise use of our land and water 
resources, protecting our fish and wildlife, pre- 
serving the environmental and cultural values of 
our national parks and historical places, and pro- 
viding for the enjoyment of life through outdoor 
recreation. The Department assesses our energy 
and mineral resources and works to assure that 
their development is in the best interests of all 
our people. The Department also promotes the 
goals of the Take Pride in America campaign by 
encouraging stewardship and citizen responsibil- 
ity for the public lands and promoting citizen par- 
ticipation in their care. The Department also has 
a major responsibility for American Indian reser- 
vation communities and for people who live in 
Island Territories under U.S. Administration. 



X'\UiM%bh'' ^jimtfw^y 



Information Circular 9239 

Multislice Mining for Thick 
Western Coal Seams 



By T. D. Hackett, D. L. Boreck, 
and D. R. Clarke 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 



^l 



o& 






Library of Congress Cataloging in Publication Data: 



Hackett, T. D. 

Multislice mining for thick western coal seams. 

(Information circular; 9239) 

Bibliography: p. 26 

Supt. of Docs, no.: I 28.27:9239. 

1. Coal mines and mining-West (U.S.) I. Boreck, D. L. (Donna L.) II. Clarke, 
D. R. III. Title. IV. Series: Information circular (United States. Bureau of Mines); 
9239. 

TN295.U4 [TN805.A5] 622 s [622'.334'0978] 89-600119 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Types of multislice mining 2 

Ascending multislice 3 

Descending multislice 5 

Simultaneous multislice ". 5 

Nonsimultaneous multislice 5 

Multislice mining with roof caving 7 

Multislice mining methods and layouts for thick western coal seams 8 

Multislice ground control 9 

Planning multislice ground control 9 

Access entries 9 

Lower slice development entries 9 

Lower slice longwall mining 9 

Location of lower slice workings 10 

Benefits of consolidated gob 11 

Geologic factors affecting multislice mining 11 

Factors affecting coal seam thickness 11 

Factors affecting competency of interburden 11 

Factors that affect compaction 12 

Lithologic composition of roof and interburden 12 

Bedding planes and abrupt lithologic changes 12 

Joints and fractures 12 

Water 12 

Multislice mining at Dutch Creek Mine 12 

Dutch Creek multislice layout 13 

Geology - Dutch Creek Mine 13 

Structural analysis of planned multislice site 15 

Cost analysis of multislice mining 17 

Description of computer model 17 

Description of hypothetical multislice cases 17 

Physical environment 18 

Mining method and plan 18 

Results of analyses 18 

Sensitivity analysis 22 

Upper split face length 22 

Number of upper split development entries 22 

Upper split longwall retreat rate 22 

Material-maintenance factors 22 

Interburden thickness 22 

Case 2 lower split development rate 22 

Productivity 24 

Resources recovery 24 

Summary and conclusions 25 

Multislice mining methods 25 

Ground control and spontaneous combustion 25 

Geology 26 

Cost sensitivity analysis 26 

Remaining problems 26 

References 26 



ILLUSTRATIONS 

Page 

1. Schematic cross section of multislice mining 3 

2. Classification of multislice mining methods 4 

3. Cross section of ascending multislice mining methods 5 

4. Cross section of descending simultaneous multislice mining methods 6 

5. Cross section of longwall caving method 7 

6. Hypothetical location for lower slice mining 10 

7. Location map of planned multislice trial in western coal seam . . . 13 

8. Layout of planned multislice trial 13 

9. Geologic column of multislice trial area 14 

10. Finite element mesh of multislice trial 16 

11. Computed stress profiles of multislice trial 17 

12. Case 1 multislice development layout 20 

13. Case 2 multislice development layout 21 

14. Costs versus upper slice face length 22 

15. Cost versus number development entries for case 1 23 

16. Cost versus longwall retreat rate 23 

17. Cost versus material-maintenance factor 23 

18. Cost versus thickness of intermediate rock parting 23 

19. Cost versus lower slice development rate for case 2 24 

20. Productivity versus upper slice face length 24 

21. Resource recovery versus upper slice face length 24 

TABLES 

1. Finite element model physical properties 15 

2. Cost assumptions common to case 1 and case 2 18 

3. Separate cost assumptions for case 1 and case 2 19 

4. Cost analysis results for case 1 and case 2 19 



UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


cm 


centimeter 


min 


minute 


ft 


foot 


pet 


percent 


ft/min 


foot per minute 


psi 


pound per square inch 


lb/ft 3 


pound per cubic foot 


St 


short ton 


m 


meter 


yr 


year 



MULTISLICE MINING FOR THICK WESTERN COAL SEAMS 

By T. D. Hackett, 1 D. L. Boreck, 2 and D. R. Clarke 3 



ABSTRACT 

Multislice mining methods were analyzed by the U.S. Bureau of Mines to determine their application 
to western United States thick coal seams; ground control, geology, and costs were considered. 
Multislice mining is used in widely varying seam conditions worldwide, including flat seams too thick to 
mine in a single pass, pitching thick seams, and seams containing a rock parting. Longwall multislice 
methods predominate, but room-and-pillar variants also exist. The initial use of the method in western 
seams is planned at a deep Colorado mine where a rock parting will be used to separate two slices 
mined by longwall. 

Ground control and spontaneous combustion are major hazards associated with multislice mining. 
A well-consolidated upper slice gob can reduce ground control problems and provide a seal against 
spontaneous combustion. Geologic analysis indicates that the consolidation of the gob depends on the 
composition of the upper slice roof, the presence of water, and sufficient overburden pressure. A 
geologically competent intermediate rock parting can also reduce ground control problems and seal 
against spontaneous combustion. 

To compete in western coal markets, the cost of multislice mining must be within the range of normal 
longwall costs. Analysis indicates that multislice operating costs should be within this range, and that 
multislice mining costs decrease as panel width increases. Relatively wide (600 to 800 ft) panels also 
provide increased coal recovery. 



Mining engineer, Denver Research Center, U.S. Bureau of Mines, Denver, CO. 

Geologist. 

Mining engineer (now with Minerals Availability Field Office, U.S. Bureau of Mines, Denver, CO). 



INTRODUCTION 



The western region of the United States contains exten- 
sive coal reserves. The Energy Information Administration 
of the U.S. Department of Energy (1) A estimates that the 
western region's demonstrated reserve base contains 
236.7 billion st, of which 141.0 billion st, or 59.6 pet, must 
be mined by underground methods. Recently, a single 
deposit containing 113 billion st, i.e., up to 182 ft thick 
and under 1,100 ft of overburden, was located by the U.S. 
Geological Survey (2) in the Powder River Basin of 
Wyoming. A significant percentage of these reserves he in 
thick seams that are too deep for surface mining and have 
a thickness that exceeds the height limits of existing under- 
ground mining equipment, requiring a portion of the seam 
to be left in the mine. The frequent occurrence of thick 
seams in Colorado, Utah, and Wyoming has been reported 
by Boreck (3). For this report, a thick seam is defined as 
a seam 15 ft thick or greater. The definition also includes 
minable seams that are split from a thick seam, or closely 
spaced minable coal seams where one of the two seams 
will be lost should standard mining methods be used. 
Because the highest longwall face being used in the United 
States as of 1987 is 14 to 15 ft high, that portion of the 
seam thickness in excess of 15 ft cannot be mined. In 
addition, thick seams frequently contain partings (and are 
termed split seams) that cannot be economically taken 
with the coal, requiring the portion of the seam above or 
below the parting to be left. It will be shown later in the 
report that coal recovery in seams with partings and in 
closely spaced multiple seams with interburden of less than 
30 ft can be greatly improved with multislice mining. In 
order to improve recovery in thick seams, the U.S. Bureau 
of Mines has investigated the multislice mining method. 

Multislice mining (also called multilift or multileaf 
mining) is the extraction of a thick coal seam in successive 



levels, usually proceeding downward in the seam so that 
later levels or slices underlay previous ones. Figure 1 
schematically shows a cross section with three slices 
worked downward in succession and some technical fea- 
tures commonly used in the method. The uppermost two 
slices work a continuous thick seam, allowing the roof to 
cave behind the faces. To provide a safe roof in the mid- 
dle slice, an artificial roof is emplaced on the floor of the 
first slice. The artificial roof, composed of wire mesh or 
other material, keeps the first slice gob from caving into 
the second slice working areas. In the lowermost third 
slice, a seam parting provides an advantageous roof layer, 
and an artificial roof is not necessary. Numerous vari- 
ations in the multislice method are possible. For instance, 
adjacent slices may be mined simultaneously, or mining of 
underlying slices may be delayed to permit gob consoli- 
dation in the upper slice. Stowing may be used to prevent 
roof caving. Also, room-and-pillar methods exist. 

Nonsimultaneous multislice mining without artificial 
roof was previously identified by Oitto (4) as best adapted 
to western mining conditions. The geologic factors af- 
fecting the development of thick and closely spaced seams 
have been discussed by Boreck (3). These factors include 
the thickness and configuration of the seam and the roof 
composition and variability. Hackett (5) analyzed the 
ground control of multislice mining. The stress on the 
upper slice gob and the consolidation of the gob were 
determined to be important for lower slice mining. Multi- 
slice layouts have been designed for an existing western 
mine (6) and, under contract to the Bureau, 5 for a hypo- 
thetical 100-ft-thick western seam. Measures to prevent 
and control spontaneous combustion have also been dis- 
cussed (7). The layout, geological factors, ground control, 
and cost of multislice mining are discussed in this report. 



TYPES OF MULTISLICE MINING 



Numerous variations of multislice mining have been 
used worldwide to get high coal recovery under local con- 
ditions of geology, subsidence control, proneness to spon- 
taneous combustion, statutes, and economics. Australia, 
United Kingdom, China, France, Germany, Hungary, 
India, Japan, Poland, Romania, Spain, Yugoslavia, and the 
U.S.S.R. have practiced or now practice multislice mining. 
Approximately 41 pet of China's coal production comes 
from multislice mining (8), and they have mined as many 
as 10 overlying slices (9). Because local conditions vary 
widely, the multislice geometry and techniques used vary 
markedly. The conditions under which the method has 



been used have often been difficult and restrictive, showing 
the technical feasibility of the method. 

Figure 2 shows one method of classifying multislice 
mining methods. Other classifications have been used, 
and not every possible multislice variation is shown. The 
branches shown lead to multislice methods that conform 
to U. S. mining practice. In the course of the discussion 
of the various multislice methods, those that have inher- 
ently high cost, low productivity, or other problems, such 
as difficult ground control, will be identified. The dis- 
cussion will be structured on figure 2, emphasizing those 
practices that conform to U.S. mining practice. 



Italic numbers in parentheses refer to items in the list of references 
at the end of this report. 



Contract H0262035, "Design and Evaluation of a Coal Mine Entry 
System For Longwall Top Slicing of Thick Coal Seams" to D'Appolonia 
Consulting Engineers, Inc. 




Figure 1. -Schematic cross section of multislice mining. 



ASCENDING MULTISLICE 

Multislice mining can proceed either downward (de- 
scending multislice, often called top slicing) or upwards 
(ascending multislice) from the bottom of the seam or 
other horizon. Ascending methods normally backfill be- 
hind the working face to provide a base for the next slice. 
Backfill is used where surface structures must be protected 
from subsidence (10-13), the overlying coal must be sealed 
to prevent spontaneous combustion (10-12, 14), when the 
seam has a massive roof that will not cave (15-17), or in 
undersea mining (11, 18). 

Ascending multislice is frequently used in thick, steeply 
dipping seams. The horizontal slicing method (fig. 2A) 
divides the coal seam into slices running along the seam 
strike and across the seam horizontally. Stowing or 



hydraulic backfill is placed during extraction to support the 
unmined coal above and provide a working floor for over- 
lying slices. In ascending inclined slicing (fig. 3B) a slice 
at the bottom of the seam is extracted, and successive 
overlying slices worked on the backfill material. A varia- 
tion on horizontal slicing, termed the ascending-descending 
method, was proposed for steep, thick seams in India (19). 
The seam is divided into blocks down the dip. Each block 
is worked with ascending slicing, and the blocks are 
worked in descending sequence. Horizontal and inclined 
slicing can also be done in descending order. Ascending 
methods have the disadvantages that the backfill does not 
completely support the overlying coal and the coal may 
fracture, causing spontaneous combustion problems in 
overlying slices. Three to four slices are reported to be 
the limit for ascending multislice (20). 



lultislice mining 



Descending 



Nonslmultaneous 



Room and pillar 



Longwall 



Sublevel caving 



Under artificial 
roof 



Retreat 




Double 
entry 




Caving 



Under rock or 
coal split 



Advancing 



Double 
entry 



Ascending 



Simultaneous 



Room and pillar 



Longwall 



Stowing 



Directly under 

consolidated 

gob 



Naturally 
consolidated 
gob 



Artificially 

consolidated 

gob 



Figure 2.-Classification of multislice mining methods. 





floor / . 



W///y/// y y/A 



Figure 3.-Cross section of ascending multislice mining 
methods. A, Horizontal slicing; B, inclined slicing. 



DESCENDING MULTISLICE 

Descending multislice methods are more commonly 
used than ascending methods because they are adaptable 
to a wider range of mining and geologic conditions (20). 
Either simultaneous or nonsimultaneous mining can be 
used. In simultaneous multislice, two or more faces follow 
each other directly in the coal seam at the same time. It 
is desirable to keep a constant distance or lag between the 
faces to avoid ground control problems (4). Nonsimul- 
taneous methods disassociate operations in the slices by 
completely extracting the overlying slice before lower slice 
mining starts. A variable period of time is left between 
the slices to keep lower slices out of the influence of upper 
slices and permit the upper slice gob to settle. Descending 
methods have a number of advantages. First, the roof is 
allowed to cave, eliminating the need for stowing. Sec- 
ondly, the roof supports and mining equipment work on 
a solid coal floor rather than having to work on weak 
backfill. 

In descending multislice, some means is required to 
provide a safe roof for the lower slice. Separating the 
slices is especially important in simultaneous multislice as 
the overlying gob has not had time to consolidate. This 
function can be accomplished several different ways. In 
figure 44, artificial roof material placed during upper slice 
mining is used to separate the slices. The lag necessary in 
simultaneous mining is indicated. An intermediate rock or 
coal band can separate the slices as in figure 45. This 
method eliminates the cost of artificial roof, but if a coal 
band is used, coal recovery is reduced, and the coal left in 
the gob may create spontaneous combustion problems. 
Upper slices have also been backfilled (fig. AC). In India 



(15), simultaneous mining with pneumatically stowed back- 
fill cemented with fly ash in the upper slice was used to 
extract a 5-m-thick seam with hard-to-cave roof. The 
methods shown in figure 4 can apply to both simultaneous 
and nonsimultaneous multislice mining. 

Simultaneous Multislice 

Simultaneous, multislice has the disadvantage of face 
interdependence (21). If one face stops for maintenance 
or ground control problems, the other face must also stop, 
totally eliminating production. Nonsimultaneous mining 
disassociates operations in the slices by completely ex- 
tracting overlying slices before lower slice mining starts. 
Faces are independent in that one face does not stop other 
faces. 

In the Karaganda Basin of the U.S.S.R., a 7.0- to 7.5- 
m-thick seam was mined by simultaneous mechanized 
longwalls in two slices, with a lag between the faces of 40 
to 60 m (22). A band of coal 0.5 to 0.8 m thick was left 
between slices to provide a roof for the lower slice face. 

An artificial roof placed while mining the top lift is 
frequently used to separate lower slices from overlying gob 
material. The roof may be laid on the upper slice support 
canopies (23) or on the floor (24). In the Miike Colliery 
in Japan (25), overlapping panels or wire mesh were laid 
on the floor in front of a support with a shortened base, 
which allows room to work. The lower face lagged the 
upper by about 40 m. Oitto (4) further discusses simul- 
taneous multislice mining in Japan. In Chinese mines, 
roof of plastic mesh is being experimented with. Because 
of the large number of slices mined, roadways are driven 
in the seam floor (26). 

Nonsimultaneous Multislice 

Nonsimultaneous multislice can be divided into longwall 
methods and room-and-pillar methods. Longwall methods 
are most common, but an interesting room-and-pillar 
method has been tested in Australia (27). A trial of two- 
slice extraction was conducted in a 5.4-m-thick seam. The 
top slice was mined using a continuous miner and a pillar 
extraction method termed the Wongawilli split-and-fender 
system. In this mining system, panels are developed by 
parallel double entries on the panel edges. Fenders be- 
tween the double entries are extracted by retreating toward 
the mains. A coal parting or septum 1.8 m thick consti- 
tuted the middle slice. The lower slice was extracted with 
a split-and-fender system, and the septum was taken down 
by raising the boom of the continuous miner. Grouted 
wooden dowels supported the septum and were used as a 
guide to indicate the correct roof level on the lower slice. 
They were inserted in the floor during top slice mining. 
The trial achieved 62 pet full seam recovery and showed it 
was possible to extract Australian thick seams using con- 
tinuous miners. Lower slice mining started after upper 
slice mining was completed, but it may be possible to mine 
upper and lower slices simultaneously. 





^ '""" 1 


^ — ^ — 

Artificial roof 

/ 


A 
Gob 


i 


-Jo'°^) 


HP 


STMm 


W/a^ //////// ^ / //////7/// / ^Ir^x 

Y///A Thick coal seal /////////// ft A/T\ 

W/////////////////y///////,^-,, \\n// t 








-jb Log ■ 



y >s -^ "^— ^ — 




Gob 


5 


\ ^ 




^ \ c s 


N ^ /"^ 


HH 


W ~T^ 


Intermediate rock or coal band 


< 


'WMMMMS, 


, J 


^ 




Figure 4.-Cross section of descending simultaneous multislice mining methods. A, With artificial roof separating slices; 6, with 
intermediate rock parting; C, with backfill in upper slice. 



Nonsimultaneous longwall methods include sublevel 
caving, stowing methods, and methods with roof caving. 
Sublevel caving is practiced in France (28), Hungary (29), 
and Yugoslavia (30-31). It can be used to extract up to a 
10-m-thick slice in one or two passes. Coal is mined by a 
combination of standard longwall and roof caving (fig. 5). 
Three meters of coal at the bottom of the slice are mined 
by standard longwall and approximately 7 m of coal in 
the roof are extracted by caving through gates in the gob 
shield or canopies of specialized face supports. The caved 
coal loads onto a conveyor in the rear of the supports, or 
directly onto the face conveyor. In France (28), the caved 
roof coal was held back by wire mesh behind the supports. 
The mesh was cut, allowing the coal to flow into a second 
conveyor between the support legs. Specialized face sup- 
ports, called banana props, were used to agitate the coal 
by raising and lowering the legs to improve drawing. 
Seventy percent of sublevel caving coal production typically 
comes from the caving operation and 30 pet from the 
longwall (32). Blasting may be necessary to destress the 
caved part of the slice. At least three overlying 10-m-thick 
slices can be worked. 

Descending simultaneous multislice longwall with back- 
filling in the upper slice and caving in the lower slice has 
been used as previously noted (15). In Czechoslovakia 
(13), the upper slice waste is usually backfilled when min- 
ing a thick seam in descending lifts. 

Multislice Mining with Roof Caving 

While stowing has some advantages in protecting sur- 
face structures from subsidence (33), it is expensive and 
labor intensive (14). Longwall methods that allow the roof 
to cave have now become the preferred technology, and 
where stowing is not required to protect surface structures 
or to achieve some other objective, would be a preferred 
method for U.S. multislice mining. Also, higher capacity 
shield supports have now replaced chock-type supports, 




« 



*o. 



w 

Via 



'<£o 



loading^ 



CO 



c3° 



c? 



o 



O 



s l ^ o I 



- Waste, bo 



Figure 5.-Cross section of longwall caving method. 



allowing the caving of massive roof that formerly required 
other methods of mining. Using caving methods, there 
exists the danger that upper slice gob could cave into the 
lower slice workings. To prevent this occurrence, some 
means is needed to either separate the slices (for example 
with artificial roof or an intermediate rock band) or stabi- 
lize the gob. 

In simultaneous descending multislice (previously dis- 
cussed), an artificial roof is used to separate the slices. 
Artificial roof might also be used in nonsimultaneous 
descending multislice, but corrosive mine water and/or 
heat from spontaneous combustion in the upper slice gob 
might deteriorate the roof material. One alternative meth- 
od used to provide stable roof is to exploit the natural 
tendency of the gob to reconsolidate. Given sufficient time 
and pressure and the right material and amount of water, 
the gob can consolidate to form a lower slice roof. Polish 
mines have used gob as a lower slice roof (34). A period 
of 3 to 5 yr passed between working upper and lower 
slices, and successful results were obtained only when roof 
rocks were within a certain range of mechanical properties. 
Reconsolidated roof is possible when the roof consists of 
argillaceous rocks, and appropriate water is present (11). 
Under optimum conditions, sufficient consolidation can 
occur after 3 months. In China, if the roof is argillaceous 
shale, and it consolidates in the presence of water, a recon- 
solidated roof for the next slice can form in 6 to 12 months 
(9, 23). 

Another alternative to artificial roof is to artificially 
consolidate the gob by injecting water combined with fly 
ash or other additives. The water promotes consolidation 
of the gob material, and the additive settles to the bottom 
of the gob to form a solid roof layer. A seal against spon- 
taneous combustion is also created. Chemical consoli- 
dation of the gob has been used in Hungary to provide a 
compacted lower slice roof (14). Cement grout, with a 
composition depending on the chemical composition and 
fragmentation of the gob, was injected through perforated 
pipe laid in floor trenches. The artificial roof produced 
was 30 to 50 cm thick and sealed the upper slice gob, 
providing a safeguard against spontaneous combustion. 
Loess mud has been injected into the gob in China, al- 
lowing extraction of more than 10 slices without occur- 
rence of spontaneous combustion (26). Mud injection can 
improve the reconsolidation of the gob and reduce the 
time necessary to reconsolidate (9). Artificial gob consoli- 
dation has also been used in Czechoslovakia (13) and in 
Japan with washery waste (27). 

Use of either artificial roof or artificial gob consoli- 
dation entails extra expense. Emplacement of the roof 
material or grout material requires extra labor and trans- 
portation underground, and the cost of roof material or 
grout must be borne. The cost for artificial roof can 
amount to 20 pet of the total coal cost per metric ton (77). 
One method to eliminate this cost and effort is to leave a 
band of rock or coal to separate the slices and form a 
lower slice roof. Where seams contain a rock split, the 
split has been advantageously used as lower slice roof. It 



is preferable to leave rock rather than coal if the coal is 
prone to spontaneous combustion. In Japan, a 1.5- to 2.0- 
m thick rock band separated simultaneous multislice faces 
at the Kushiro Colliery in Hokkaido (21). Forty-five units 
(upper plus lower slice), making up 90 faces, were mined 
with this method. The use of a coal band or septum to 
separate room-and-pillar slices in the Australian 
Wongawilli system was previously discussed. In the 
U.S.S.R., a 0.5- to 0.8-m band of coal separated simulta- 
neous multislice faces (22). A 3-m-thick rock split will 
form the lower slice roof in a planned trial of nonsimul- 
taneous multislice mining at the Dutch Creek Mine near 
Redstone, CO. 

MULTISLICE MINING METHODS 

AND LAYOUTS FOR THICK 

WESTERN COAL SEAMS 

If multislice mining is to be used in the United States, 
it must conform to American economic, safety, and legal 
requirements. Cost is a primary consideration because if 
costs are not competitive, the method will not be used. 
Many of the multislice methods used in other countries 
would not be cost competitive because of extra labor and 
material requirements or poor productive capacity. Im- 
provements in technology, such as mechanized placement 
of artificial roof, may reduce the labor requirements and 
costs for some methods. The following discussion is di- 
rected primarily to flat-lying, thick seams. However, steep 
thick seams exist in the Grand Hogback area of Colorado 
(35), and multislice operations adapted to steep thick 
seams may have application. 

Descending, nonsimultaneous, longwall, which allows 
roof caving and uses an intermediate rock band as the 
lower slice roof (fig. 42?), may be the multislice method 
best suited to U. S. mining requirements. Ascending mul- 
tislice requires expensive stowing material and placement 
systems, which descending multislice does not require. 
Nonsimultaneous longwall has the advantage of separating 
the slices in time, allowing the upper slice gob to con- 
solidate and reducing the interference of mining operations 
in one slice on the other slice. Longwall with roof caving 
has become the preferred method worldwide and is stan- 
dard practice in the United States. Artificial roof can 
provide good lower slice conditions, but is expensive and 
slows retreat of the face. Recent developments in mecha- 
nization of roof laying may reduce the cost of artificial 
roof, but if the artificial roof can be eliminated altogether, 
costs will be even lower. A rock band offers a relatively 
cheap method to separate the slices. The method has 
been successfully used abroad, and applicable conditions 
exist in the United States. 

In figure 2, the heavy black line leads to multislice 
variants that may have application in thick, relatively flat 
western coal seams. Retreat and advancing longwall meth- 
ods are indicated. Retreat is the most commonly used 
longwall method in the United States. Advancing long- 
wall is currently being used in one mine in Colorado (36) 



where the top 3 m of a 6-m seam is being mined, leaving 
a 3-m-thick intermediate rock band. Multislice mining 
underneath the rock parting is planned (37). 

A major difference between United States and foreign 
longwall practice is the type of entry system used to devel- 
op the longwall panel. Head-tail entries in countries other 
than the United States are typically single entries. In the 
United States, a minimum of two entries must be used, 
and three- and four-entry systems are the most common. 
American longwall development entries also differ in cross 
section and support. A rectangular entry section with a 
flat roof and roof bolts are used, rather than a semicircular 
cross section with arches. In the United Kingdom, rec- 
tangular entries are sometimes used, especially in retreat 
mining. 

If standard U.S. longwall practice can be adapted to 
multislice mining, introduction of the method will be easier 
than nonstandard practice that does not conform to U.S. 
legal and safety requirements is used. Two- and three- 
entry systems are the standard longwall development sys- 
tems in the western States where thick seams amenable 
to multislice mining exist. They provide acceptable cost, 
good ground control conditions, adequate ventilation cross 
section, adequate room for belts, and access for rubber- 
tired man trip vehicles and supply vehicles. They also 
conform to U.S. legal requirements. Single-entry systems 
avoid some ground control and spontaneous combustion 
problems and have been recommended for multislice 
mining of very thick coal seams. The remnant chain pil- 
lars left by multiple entries can cause stress concentra- 
tions in underlying slices and contribute to the occurrence 
of spontaneous combustion, especially if the pillars are 
crushed. However, single entries are not currently legal in 
the United States. Congestion and equipment interfer- 
ence, entailing a loss in productivity, can occur, and there 
may be insufficient room for rubber-tired man trip vehicles 
and supply vehicles. 

A multislice mining operation has been designed for a 
hypothetical 100-ft-thick western coal seam. The complex 
design incorporates single-entry pillarless mining for long- 
wall development and ten 10-ft-high slices. More than 
100 yr would be required to extract the full seam thickness. 
To provide long-term stability required to keep the mine 
openings accessible for 100 yr, the mains were located in 
the seam floor. The method proposed is similar to 
Chinese multislice mining in thick seams, where main 
headings are driven in the floor of the coal seam, and a 
minimum of protective pillars are left for pillarless mining 
(9). An alternative to multislice mining of a 100-ft seam 
might be sublevel caving. Up to 10 m can be extracted in 
one combined longwall and caving slice (30), possibly 
reducing the required number of slices from 10 to only 3 
or 4. 

A nonsimultaneous multislice operation has been de- 
signed for an existing 500-ft-deep thick seam in Utah (6). 
Two-entry longwall development was selected for both the 
upper and lower slices. A 3.5-ft-thick coal or rock parting 
would be left to separate the 7-ft slices. A modified 



two-entry system was planned for the lower slice. The 
lower slice chain pillars were designed wider than the 
upper slice chain pillars, permitting the lower slice 
development to be used for adjacent lower slices. Lower 
slice entries were located (inset) 85 ft inside the mined- 
out panel of the upper slice. Access entries between the 



mains and lower slice were designed to pass directly 
underneath the mains barrier pillar. Similarly, lower slice 
crosscuts would pass directly under upper slice chain 
pillars. The expected stress concentration underneath the 
upper slice chain pillar was considered low enough to 
provide adequate crosscut stability. 



MULTISLJCE GROUND CONTROL 



Poor ground control constitutes both a safety hazard 
and a major cost. Roof falls continue to be a major cause 
of mining accidents, and the cost of cleaning up and resup- 
porting roof falls is high in terms of labor and lost pro- 
duction. In a multislice operation, should a roof fall occur 
in the lower slice roof and propagate into the upper slice 
gob, it might create a severe hazard and possibly result in 
loss of the lower slice face. 

For an experimental operation such as multislice min- 
ing, it is desirable to get the best possible ground control. 
One approach to achieving this objective would be to 
locate multislice workings, as much as possible, in areas 
where better ground control is expected. A multislice 
mining plan based on the expected locations of good and 
poor ground control would greatly improve the chances of 
success of the operation. 

PLANNING MULTISLICE GROUND CONTROL 

Multislice mining consists of three stages, each of which 
must be accomplished in sequence to successfully get the 
coal out of the mine. These three stages experience dif- 
ferent strata stresses and conditions, and separate ground 
control plans are needed for each. The first stage is to 
access the lower slice. Access entries and roof support 
must contend with upper slice abutment stresses. The 
development of the head-tail entries, if retreat mining is to 
be used, is the second stage. The condition of the lower 
slice roof and time are major ground control factors to be 
dealt with. The third stage is longwall mining of the lower 
slice. The condition of the lower slice roof is again a 
major factor. 

The following discussion pertains to the lower slice of 
a multislice operation. Because multislice mining has not 
been done in the United States, the discussion is nec- 
essarily hypothetical. 

Access Entries 

If more than one slice is to be mined, an extremely 
complex access entry system may be needed, as is the case 
for the 100-ft-thick seam extracted in 10 slices. As the 
number of slices increases, interaction between the slices 
and ground control problems also build up. If only two 
slices are to be extracted, the access entry system can be 
simpler, and ground control problems should be fewer. 

To reach the lower slice, the entries must pass under- 
neath the barrier pillar between the mains and the panel, 
or the chain pillars between upper slice panels. Both 



structures sustain the upper slice abutment stresses, which 
will also load the access entries where they pass under- 
neath. The increased stress under the abutment and resul- 
tant fractures in the floor material may cause squeeze, 
floor heave, or roof instability. Ground control problems 
may be increased by the requirement that the access en- 
tries remain open and safe for the entire life of the lower 
slice panel. Because the lower slice entries are below 
existing grade, water may collect there and cause ground 
control problems and bog down equipment. 

Lower Slice Development Entries 

Lower slice head-tail entries will need to remain open 
and safe for the period of time required to develop and 
mine the lower slice panel, if retreat longwall is used. If 
advancing longwall is used, the entries must remain open 
for the period of time necessary to advance and recover 
the panel. Good roof stability needs to be maintained 
during that period. Planning factors to be considered 
include the stress on the entry system and the condition 
and strength of the lower slice roof. 

If the lower slice entries are located in a destressed 
zone, as previously discussed, this is beneficial. However, 
a better understanding of gob stress and site-specific infor- 
mation will be needed to predict actual stresses on entry 
systems at future multislice sites. 

In the western United States, the first multislice opera- 
tions will probably work under a seam parting rather than 
under artificial roof or directly under consolidated gob. 
The thickness, geology, and condition of the parting will 
have an effect on the stability of the lower slice roof. 
These factors are discussed in a later section of this report. 

Lower Slice Longwall Mining 

The stability of the roof is a major consideration during 
this stage of mining. If a roof fall occurs between the 
face-support canopy and the face, there is the danger that 
it may propagate into the overlying gob material. Thus, 
consolidated gob is desirable to limit the extent of the fall. 
If a rock parting is used to separate the slices, the con- 
dition and stress of the parting are important. Naturally 
occurring fractures, such as joints (discussed later), may 
decrease parting strength. It is possible that fractures may 
be induced by upper slice mining. Because of the over- 
lying gob, roof action may be different than in standard 
longwall. Hypothetically, the weaker gob and parting 
should not be able to sustain large spans of hanging roof 



10 



behind the face supports. Water may be present in the 
gob and cause wet conditions on the lower slice face. 

LOCATION OF LOWER SLICE WORKINGS 

Ground control conditions on the lower slice will be a 
major factor in determining the success or failure of the 
multislice operation. Thus, the lower slice needs to be 
located and laid out to provide a safe and productive 
ground control environment. Any feature that increases 
stresses on lower slice workings, or weakens lower slice 
roof, can result in poor ground control. Widespread min- 
ing experience has shown that high stresses and poor min- 
ing conditions are usually encountered under remnant 
pillars, whereas good conditions are encountered under 
gob (5). Unmined pillars in overlying seams can transfer 
load concentrations to underlying workings (38) and result 
in bumps (39). These observations, with associated ground 
control theory, can form a logical basis for locating lower 
slice workings and identifying areas of potentially poor 
ground control. 

When a longwall panel is mined, a portion of the load 
originally carried by the panel coal is transferred to the 
abutments because of the poor load-bearing capacity of the 
gob; a pressure balance exists between the abutments and 
the gob (40). The result is high abutment stresses on the 
panel edges and ends, with a corresponding destressing of 
the gob. Figure 6A shows a hypothetical stress profile 
across a longwall gob, far removed from the panel ends. 
The actual stress profile depends on the abutment pillar 
stiffness, the panel width, the presence of massive beam- 
forming strata in the overburden, and the properties of the 
gob. Close to the abutments, a destressed zone usually 
exists where the vertical stress on the gob is less than 
original cover stress. The gob stress rises towards the 
panel center, possibly reaching original cover stress at 
three-tenths of the cover depth behind the longwall face, 
if the panel is wide enough (40). 



400 



High abutment 
stress 




Figure 6.-Hypothetical location for lower slice mining. 
A, Stress profile across upper slice gob; 8, window of favorable 
conditions for lower slice workings. 



The shape and magnitude of the stress profile indicate 
a logical location for the lower slice longwall. Lower slice 
ground control conditions are likely to be better where 
vertical stress is reduced and worse in high stress zones. 
The destressed zone beneath the upper slice gob is likely 
a good location for lower slice development, whereas 
ground control problems might be expected beneath the 
highly stressed abutment zones on the panel edges. An 
additional consideration is the consolidation of the upper 
slice gob. Where stress on the gob is very low, the gob 
may not be consolidated, thus the destressed zone imme- 
diately next to the abutments is also a likely location to 
avoid placing lower slice development entries. Lower slice 
entries can be inset far enough from the gob edge to avoid 
this zone, but not so far as to reach the zone where full 
overburden pressure exists. Hypothetically, there exists a 
window of optimum destressing and gob consolidation 
conditions where lower slice development entries would be 
best located. The location and width of the window would 
depend on the upper slice gob stress profile, consolidation 
of the gob, and the capabilities of the development support 
system. Figure 6B shows the general location of the win- 
dow that might be favorable for lower slice workings. 

Insetting of lower slice entries from the edge of the 
upper slice gob has been practiced in the United Kingdom, 
U.S.S.R., China, and Japan. At the Daw Mill Mine in the 
United Kingdom, lower slice entries were offset (inset 
from upper slice entries) 4.5 to 14 m (6, 41). Upper slice 
single entries were supported by steel arches, whereas 
lower slice single entries were supported only with square- 
set supports and showed no evidence of weight. Lower 
slice gates (development entries) were offset from upper 
slice gates in the Kostenko Mine in the U.S.S.R. to mini- 
mize problems of strata interaction (22). In the U.S.S.R., 
a requirement for thick-seam mining is the location of 
lower slice workings under upper slice gob not more than 
5 to 7 m from the edge of the upper slice pillars (42). 
Pillarless mining is a technique practiced in China to mine 
thick seams (8). One measure in pillarless mining taken 
to simplify gate maintenance is to locate the gate in a 
stress-relieved area (9). Japanese practice at several mines 
was to recess lower slice entries inside the upper slice 
entries to place them under gob (4). The strategy has the 
disadvantage that succeeding lower slices will become 
narrower and narrower, reducing recovery. 

Other types of layouts have been suggested for the 
lower slice. Bise (43) suggested driving the lower slice 
gateroads outside the boundaries of the upper slice panel 
to place them beyond the zone of abutment pressure. This 
layout has the advantage of placing entry roof under undis- 
turbed coal rather than under the possibly cracked floor of 
the upper slice panel. Wilson designed a lower slice layout 
for an existing thick-seam mine in Utah (6). Lower slice 
gateroads would be located beneath gob, but a crosscut 
passed beneath the upper slice gateroads and abutments to 
reach the adjacent lower slice. Mining beneath the upper 
slice abutment opens some layout options to mine de- 
signers, but if abutment pressures are high, it may not be 



11 



feasible. Individual designs will probably be needed for 
each thick-seam deposit, depending on site-specific 
parameters. 

BENEFITS OF CONSOLIDATED GOB 

Allowing time for consolidation of the upper slice gob 
can reduce the risk of the gob caving into lower slice de- 
velopment or longwall face workings. If the intermediate 
rock or coal band separating the slices was to become thin 
or to fail, upper slice gob might be directly exposed in the 



lower slice roof. Given sufficient time, overburden pres- 
sure, some water, and roof composition, the gob may 
regenerate as previously discussed. A reconsolidated gob 
will also aid in sealing off any spontaneous combustion 
heating that may have occurred in the upper slice gob. 
Thick western coal seams may be more prone to spon- 
taneous combustion than thinner eastern seams. Mining 
underneath a spontaneous combustion heating in the upper 
slice gob would be extremely hazardous. Additionally, 
allowing time for gob consolidation will reduce interaction 
with adjacent longwall panels. 



GEOLOGIC FACTORS AFFECTING MULTISLICE MINING 



The geologic factors that affect development of multi- 
slice mining include many of the same factors that will 
affect the development of standard longwalls (3). Features 
that may affect development include the lithology of the 
roof and floor rock, the thickness of the coal seam, the 
presence, development, and composition of partings in the 
coal, the degree of development of cleats and joints, the 
presence of major and minor faults and fracture zones 
cutting the deposit, and the presence of undulations in the 
seam. 

At present, no operations in the West are using the 
multislice mining method. Most of our knowledge has 
been derived from case studies of foreign operations. As 
such, the effects of these geologic factors on western thick 
seam development are theoretical, and will not be verified 
until the multislice method is used to mine western thick 
seam deposits. 

The geologic factors that potentially affect multislice 
longwall mining can be categorized in three main divisions: 
(1) the factors that directly or indirectly limit the thickness 
of coal in the upper and lower slices; (2) the factors that 
decrease or otherwise affect the competency of the inter- 
burden left between the two slices; and (3) the factors that 
affect compaction of the gob. 

FACTORS AFFECTING COAL 
SEAM THICKNESS 

Thickness of a seam or of separate coal seams is an 
important consideration. During panel development, the 
minable thickness is determined by the mine plan and 
mining equipment used. A decrease in thickness below a 
minimum determined by the equipment limits the reserves 
accessible to the company unless rock is mined. The de- 
crease can be an actual thickness loss where the seam 
either thins out abruptly, is faulted out, or is eroded. A 
reserve loss can also be caused by undulations in the seam 
or by partial displacement of the seam by faulting. This 
condition forces the equipment out of the coal and into the 
roof and floor rock. 



In western coal, a seam can thin, thicken, or split over 
a short distance. A representative example of this can be 
found in Collins (44) in the discussion of the Coal Basin 
Coalbed of the Carbondale Coalfield, western Colorado: 

"In the Bear Creek area (Sec 21,T.10 S,R.89 W) four 
distinct beds are present, from bottom to top 2 feet, 
3 feet, 2 feet, and 10 feet thick, separated by partings 
5 feet, 1 foot, and 1 foot respectively. In the 4th 
North entry of the L.S. Wood mine (SW 1/4 Sec. 8), 
a single seam approximately 25 feet thick is present, 
while less than one-half mile north, along the south 
fork of Coal Creek, three beds appear, 3 feet, 6 feet, 
and 8 to 10 feet thick, separated by partings 3 to 4 
feet and 4 to 6 feet in thickness. West of the old 
Coal Basin townsite, the seam again appears as a 
single bed approximately 30 feet thick." 

From maps given in the report, the linear distance repre- 
sented in the discussion was estimated to be approximately 
4 miles. By written communication from the author, an 
error in the original paper reports the thickness of the coal 
to be 35 ft. The correct thickness is 25 ft. 

The thickness and any changes in thickness are often 
the direct result of the deposition of the coal environment. 
During initial peat accumulation, the sedimentation pro- 
cesses dominant during deposition affect the final form of 
the deposit. These processes that control the thickness 
and change in thickness are discussed in detail in the work 
of Ryer (45), Lawrence (46), and Flores (47). 

FACTORS AFFECTING COMPETENCY 
OF INTERBURDEN 

The interburden is the material, either rock or coal, 
that separates the upper and lower slices. It acts as the 
roof for the developing lower slice and separates the up- 
per slice gob from the lower face. For this reason, the 
strength of the plate of rock or coal making up the inter- 
burden is critical to the success of a multislice operation. 



12 



The interburden may also act as a seal, preventing gas, 
water, and finer material from moving between the dif- 
ferent slices. The low permeability may prevent air leak- 
age between the two panels, decreasing the potential of 
developing spontaneous combustion in the gob or fractured 
coal in the two slices. 

Factors that will affect the stability of the interburden 
include its thickness and lithology. These, in part, will 
determine the strength of the material separating the two 
slices. Both thickness and lithology can vary significantly 
in a short distance across a panel, resulting in variations in 
the strength and stability of the plate at different places 
along the face. 

Along with the above, the presence, development, and 
continuity of bedding planes, cleats, joints, and frac- 
tures are also important. These may adversely affect the 
strength of the interburden by acting as potential planes of 
failure. They may also act as conduits, allowing gas and 
water to move between the two slices. These factors can 
also change over a short distance. 

FACTORS THAT AFFECT COMPACTION 

The gob, how well it compacts, and how long it takes to 
compact are important considerations in multislice mining 
for several reasons. First, if the gob in the upper panel is 
well consolidated, then a roof fall initiating in the split is 
less likely to propagate into the gob. Any breaks that 
propagate through the interburden would allow unconsol- 
idated material to cave into the lower workings. Like the 
interburden, the consolidated gob material may form a 
barrier, limiting the transfer of gas, water, and ventilation 
air between the two panels. Finally, the time factor is 
important, especially in nonsimultaneous mining where 
extraction of the lower panel is dependent on compaction 
of the gob from the upper panel. 

Several of the major geologic factors that are hypoth- 
esized to control gob compaction are lithology of the roof 
and interburden, presence of bedding planes and abrupt 
changes in lithology, joints and fractures, and water. 

Lithologic Composition 
of Roof and Interburden 

The lithology of the roof of the upper slice and the 
interburden between the two slices affects the compact- 
ability of the gob for both the upper and lower panels. 



The lithology often varies significantly, both vertically 
and laterally. As the lithology is an important factor 
determining the strength of the roof and split, it will also 
determine the strength of individual blocks that makeup 
the gob. Ultimately, the lithology will be a main factor in 
determining the characteristics of the gob, including its 
compactibility. 

Bedding Planes and Abrupt 
Lithologic Changes 

Bedding planes and surfaces of lithologic change (such 
as an erosional surface) can often act as planes of sepa- 
ration and failure during caving. As such, the number and 
degree of development of these horizontal to low angle 
features are important. The spacing between the more 
well-developed bedding planes in many cases may equal 
the smallest dimension of the gob block. 

Joints and Fractures 

As joints and fractures also represent surfaces of poten- 
tial failure, their continuity and spacing are important in 
determining the size and shape of individual gob blocks. 

Water 

Water may aid in the compaction process. In the pres- 
ence of water, some argillaceous roofs may consolidate 
more readily, requiring less time for compaction (23). 
Yet, too much water may actually decrease stability of the 
consolidated roof (34). Water also has been known to 
collect in mined out areas in the upper slice, eventually 
rushing into the lower working face and creating an ex- 
treme hazard. 

In summary, the geologic factors that affect multislice 
mining are those that affect the minable thickness of the 
coal, the strength and permeability of the interburden 
separating the two slices, and the strength and compact- 
ibility of the gob. In any given depositional environment, 
these features can change within a short distance. Given 
this, the conditions on the face can also change quickly. 
Predicting the effects of these factors on panel layout and 
development in a multislice operation requires careful 
mapping of both the lithology and structure of the coal- 
bearing section. 



MULTISLICE MINING AT DUTCH CREEK MINE 



A joint Bureau-industry test of multislice longwall (37) 
is planned at the Dutch Creek Mine (formerly Dutch 
Creek No. 1 and No. 2), operated and owned by Mid- 
Continent Resources, Inc., near Redstone, Colorado 
(fig. 7). The mine extracts high-grade metallurgical coal 
from two seams separated by about 500 ft. Multislice 



mining is planned in the lower of these two seams, which 
is split into two sections by a rock parting approximately 
10 ft thick. Only the upper section, designated the B bed, 
is now mined. Lower slice mining is planned in the A bed, 
underneath the rock parting. 



13 



DUTCH CREEK MULTISLICE LAYOUT 

The upper slice, a longwall panel designated LW102 
(fig. 8), is an 800-ft-wide advancing longwall utilizing 
monolithic pack wall supported double-entry head-tail 
entries (36). The lower slice will use a two-entry develop- 
ment system inset approximately 60 ft inside the upper 
slice pack walls on both sides. Double entry was picked 
for the lower slice because it has no four-way intersections 
that might increase roof problems, and because it provides 
better recovery than a three-entry system. Unlike the 
upper slice, the lower slice will use retreating longwall, 
requiring full development prior to mining. Retreat long- 
wall will permit probing lower slice ground control con- 
ditions and blocking out of the panel prior to committing 
the longwall face equipment. The lower panel width will 
be 510 ft, giving a combined upper and lower slice re- 
covery of approximately 68 pet. 

Access to the lower bed will be by a ramp in the ex- 
isting upper slice head-tail entries. After the ramp has 
reached the lower bed, development entries will pass under 
upper slice gob until the required 60-ft inset is obtained. 
Development entries are not planned to be driven through 
or underneath the upper slice barrier because it is highly 
stressed from upper slice mining and might bump. The 
high stress would also cause severe ground control prob- 
lems while driving through the barrier, especially near the 
edge of the pillar. 

GEOLOGY - DUTCH CREEK MINE 

The Mid-Continent Dutch Creek Mine is located in the 
Coal Basin area of the Carbondale Coalfield on the south- 
eastern edge of the Piceance Basin in Pitkin County, CO 
(fig. 7). The mine produces high quality metallurgical coal 
from the Upper Cretaceous Williams Fork Formation of 
the Mesaverde Group. 

The coal-bearing strata outcrop in the Coal Basin 
Anticline, a predominate structure in the area. The anti- 
cline, a large-scale fold plunging to the northwest, was 
believed to have been formed by doming over a small 
laccolithic intrusion (48). The structure has been deeply 
eroded, exposing the Williams Fork Formation. At the 
mine, the strata strike north-northwest and dip approxi- 
mately 11° to 13° to the southwest (49). 

The Mesaverde Group consists of the lies and Williams 
Fork Formations. The lies Formation contains the tongue 
of Mancos Shale and the Rollins Sandstone Member at the 
top (44). The Williams Fork Formation is divided into the 
Bowie Shale Member, the Paonia Shale Member, and an 
undifferentiated unit. The coal seams of greatest eco- 
nomic importance are from the Bowie Shale Member. 
The Coal Basin A and B seams (referred to as the A and 
B seams hereafter) are located at the base of the Bowie 
Shale directly above the Rollins Sandstone Member. 




Figure 7.-Location map of planned multislice trial in western 
coal seam. 



LW 101 



jQ 



Packwalls 

LW 102 toil entry 



D 

ID 



\z 



m en en en en en en en en pn en 



t 



LW 102 L development 



LW 102 



:n en en en en en en en en en en 




en en en en en en en en en en en en en en 



LW 103 L development 

KEY 

— Existing development 

— Plonned development 




200 



Scale, ft 



Go 



^vV<,°.c"S(>°iSri'..^;V. | Jpper slice ,; 



„i, i wi'ni°°£»» - C°r Access ramp to ao h i Wl02f 

,°. b .i:ff Sk-ft<aS _Pockwolls i/> t V 1( lower slice .,;■; ',5° -v.-. v'^" 



Rock partinc 



Nock p 
fA beai 



25 50 

i i 

Scale, ft 



Lower slice LWI02L 
development entries 



Figure 8.-Layout of planned multislice trial. A, Map view; 
B, cross section of access area parallel to longwall face. 



14 



The coals of the Coal Basin area have been described 
by Collins (48) as being deposited generally in fresh-water 
swamps. They are made up primarily of the remains of 
woody plants. The coal in the study area has been up- 
graded to medium volatile bituminous in rank. Although 
the coal measures have been cut by a number of dikes and 
sills, their presence is not believed responsible for the 
increase in rank of the coal. Instead, heat from the lac- 
colith is believed to be responsible for upgrading the coal. 

At present, there is a limited amount of published in- 
formation available on the geology of the Dutch Creek 
Mine. The following discussion was derived from Collins 
(44, 48), Bigarella (50), and from work and observations of 
Bureau staff. A number of geologic factors will be im- 
portant in determining the success of a multislice operation 
at the Dutch Creek Mine. These are variation in thickness 
of the A and B coal seams, variation in thickness of the 
split separating the two seams, and lithologic variation in 
the roof, floor, and interburden between the two seams. 
The presence of cleats, joints, and fractures are also im- 
portant. The frequency and degree of development of 
joints and fractures in the interburden rock will determine 
the strength and permeability of the material separating 
the two slices. 

Figure 9 is a composite log of core taken in the roof 
and floor of the B seam; the core was taken in the head- 
gate entry of the panel being analyzed (panel LW102). 
Seam thickness at the panels is approximately 10 ft for 
both the A and B seam. The thicknesses of the seams are 
not believed to change rapidly in the study area, mini- 
mizing the potential for problems due to thinning of the 
seam below the limit set by equipment. The interburden 
was reported to vary from 4 to 12 ft thick in the mine. In 
the study area, the interburden is approximately 10 ft thick 
(fig. 9). The face appearance of the coal in the Coal Basin 
area is variable (50). The coal can be blocky, shattered, 
and any degree in between the two. The Coal Basin Seam 
was reported to often be uncleated. Yet, after failure 
during testing, visually uncleated samples showed develop- 
ment of two cleat sets. Observations in the mine show 
that the coal is not really uncleated, but it has been 
masked by subsequent fracturing and shearing in numerous 
directions related to tectonic activity (51). 

The lithology of the roof, parting, and floor rock is 
variable (52). The predominant lithologies present are 
sandstone, siltstone, mudstone, and coal. From figure 9, 
the immediate roof of the coal deposit consists of siltstone 
and black shale, coarsening upward into light gray sand- 
stone. Above the sandstone, the roof is made up of shale, 
with minor amounts of sandstone and clay. Upon observa- 
tion, the immediate roof appears highly competent, with 
bed separation similar to that of slate. Few fractures were 
noted in the roof. 

The floor of the A seam is made up of shale, carbona- 
ceous shale, and siltstone. In the core taken at the study 
area, the separation between the A seam and the 



underlying Rollins Sandstone is approximately 7 ft thick 
(fig. 9). This sandstone unit has in the past been a source 
of both water and gas that has migrated into the upper 
slice workings. Development of the lower slice in the A 
seam is not expected to present a problem because of the 
degassification caused by mining of the upper slice. 

The parting separating the A and B seams is critical as 
it is the floor of the upper slice and the roof of the lower 
slice. At the study area, the parting was made up of black 
silty or carbonaceous shale with coal streaks, silty sand- 
stone, and interbedded sandstone and shale (fig. 9). A 
section of the interburden was exposed due to heave along 
the tail entry of one of the panels. The exposure consisted 
predominately of thin-bedded sandstone interlayered with 
shale. Bedding planes were common and closely spaced in 
both the core and exposure of the parting. 

Jointing and fracturing of the rock was common in the 
core. Most of these planes of weakness were found in the 
interburden. The core was cut by a number of high angle 
to vertical fractures in the parting rock. It is not known 
if these fractures were naturally occurring or the result of 
heave. 




20 



-ihio 

o 



Sandstone 

Shale with some sandstone 

Sandstone with 
carbonaceous stringers 

Sandstone 
Carbonaceous siltstone 

Coal B 

Shale with coal streaks 
Sandy shale with coal streaks 

Coal A 

Shale with coal stringers 

Clay 

Sandstone with 
carbonaceous stringers 



Figure 9.-Geologic column of multislice trial area. 



15 



The geologic factors that will affect the success of multi- 
slice mining in the study area are those that will affect 
either the strength of the interburden between Coal Basin 
A and B seams or the compaction of the gob. We do not 
know how the lithology and relative strength of the inter- 
burden vary across the panels. But, if the core section is 
representative of the characteristics of the split over the 
lower panel, the strength of the interburden between the 
two seams could be decreased because of closely spaced 
bedding planes and high angle to vertical fractures present 
in the strata. 

Due to the competency of the roof rock making up 
the individual gob blocks, the upper slice gob is not ex- 
pected to compact readily. It may take several years to 
consolidate. 

STRUCTURAL ANALYSIS OF PLANNED 
MULTISLICE SITE 

A simplified two-dimensional plane strain finite element 
analysis of the multislice trial site was made to estimate 
the stress profile across the site upper slice panels. The 
computer program Automatic Dynamic Incremental Non- 
Linear Analysis (ADINA) (52) was used for the analyses. 
ADINA was selected because of its capability to model the 
complex longwall and pack wall geometries and to model 
formation of pack walls through the birth-death procedure. 
The determined shape and magnitude of the stress profiles 
indicate the premining stress environment for the lower 
slice and can aid in locating lower slice head-tail entries. 

Some computer model input parameters were simplified 
to reduce costs and provide conservative estimates. The 
seam was modeled as flat rather than at the actual 10° dip. 
A uniform depth slightly in excess of the actual overburden 
thickness was input. A single gob modulus was used for 
the entire gob rather than dividing the gob into zones of 
different moduli as done by other researchers (53). A 
previous analysis (5) using the model showed that the 



upper slice gob modulus input into the model determined 
the amount of gob destressing and the abutment stresses. 
A final estimate of 60,000 psi was made for the gob 
modulus and the results reported herein for that input. 

The finite-element mesh (fig. 10) modeled a vertical 
section parallel to the longwall face far removed from the 
face ends. The model incorporates the upper and lower 
sections of the coal seam, each 10 ft thick, and the 10-ft 
rock interburden. The cover depth is 3,000 ft, and the 
widths of the pack walls, entries, and two adjacent long- 
walls designated LW101 and LW102 are 7, 16, 550, and 
800 ft, respectively. The mechanical properties of the 
coal, rock split, pack walls, and roof-floor strata are sum- 
marized in table 1. The materials modeled are assumed 
to be linear-elastic and isotropic. The mesh consists of 
1,760 nodes and 1,700 quadrilateral elements, and is 
2,700 ft wide by 5,900 ft high. 

The element birth-death option available in ADINA 
was used to simulate panel extraction and the subsequent 
formation of gob. This feature enables the user to 
activate-deactivate designated groups of elements. The 
geometry of the gob zone was defined to allow the birth- 
death option to operate on the elements within the gob 
boundaries. The upper limit of the gob zone is controlled 
by the caving height (the height to which block-forming 
fractures propagate into the immediate roof), and is as- 
sumed to be four times the seam height (fig. 11). The 
caving height was assumed to be four times the seam 
height because the Mid-Continent roof is known to break 
into large blocky fragments, which would probably give a 
low-bulk factor (the ratio, volume of gob to volume of 
original roof rock) (about 1.25). A bulk factor of 1.25 
would produce a caving height of four times seam height. 
Initially, the elements within the specified boundaries 
represent the coal seam and immediate roof. To simulate 
excavation, the gob zone elements are deactivated (death), 
and are then assigned assumed gob properties and re- 
activated (birth), simulating the gob. 



Table 1. -Finite element model physical properties 



Layer 


Rock type 


Compressive 

strength, 

psi 


Composite 

Young's 

moduli, 10 6 psi 


Composite 

Poisson's 

ratio 


Roof: 

Upper 

Middle 


Siltstone-sandstone 

. . do 

Fine sandstone 


19,897-29,610 
15,774-23,799 
24,767 
1,771-6,147 
4,108- 6,847 
5,867-32,931 
1,615-3,343 
4,068-22,962 


3.523 
3.771 
3.440 
1 .476 
2 .253 
3.039 
'.526 
1.209 


0.182 
.208 


Lower 


.189 


B seam 

Pack wall 


Coal 

Concrete 

Siltstone-sandstone 

Coal 

Shale-coarse sandstone . . 


.32 
.13 


Rock parting 

A seam 


.185 
.32 


A seam floor 


.203 



Average. 

2 Not composite, 10 pet of average modulus of 2.529 x 10 6 psi. 

NOTE.-Physical properties were determined from uniaxial compression tests of cores. 



16 



SURFACE 



2,000 



l±J 

< 
ti- 
er 

CO 



3,000 



Q- 
UJ 
O 



3,900 



5.900 




500 1,100 1,435 1,812 2,100 

HORIZONTAL DISTANCE, ft 



2,700 



KEY 

A B 

D o Area enlarged 



Coarse sandstone with 
carbon partings 


A 




























B 


Sandstone-si Itstone 
interbeds 
































































Composited sandstone and 
sandstone-siltstone interbeds 




























































































































■ 






".-■■'; - ■ 


Composited sandstone and 
siltstone 


















































:"■■ 


•■' , 




-„| 
















































, i 








' "^> 
















:•; 


I 


t;i 




■V* 


...-.Wc 


■;'■ !■'•■ 


Coal B 
























Lj 








































































Sandstone-siltstone 
interbeds 














































































































Coal A ^M?" 




Coarse sandstone with 
carbon partings 


D 




























c 



KEY 

Caved 

material 

Concrete 



D 

I -• : I packwall 
1 1 Coal 



Figure "lO.-Finite element mesh of multislice trial. A, Global mesh; B, detailed mesh. 



17 



Figure 11 shows the stress profile across the two panels 
(LW 101 and LW 102) for one and two panel extraction. 
The second panel modeled (LW 102) is the site of the 
planned multislice mining. The initial overburden stress of 
3,300 psi is shown for reference. After extraction of one 
panel, the abutment stress on the pack walls between the 
panels is approximately doubled. The stress on the mined 
out panel is only 33 pet of the original cover load. After 
extraction of two panels, the abutment stress rises from 
four to five times cover stress, and the stress on the second 
panel (LW 102) gob, which is to be the upper slice, is 
about 43 pet of the cover stress. 

The model indicates that the vertical stress on the lower 
slice (LW102L) will be substantially less than the cover 
stress of 3,300 psi. The planned 60-ft inset should locate 
the lower slice development entries sufficiently far from 
the upper slice pack walls to avoid the high stress concen- 
tration on the pack walls. The development of lower slice 
entries underneath upper slice head-tail entries as pro- 
posed for another site (6), probably would not be practical 
at the Dutch Creek site. 



17.5 



15.0 



12.5 



1 10.0 — 



"i r 



KEY 
LW 101 mined 
LW 102 and LW 102 mined 




DISTANCE 



l 1 Panel 

Panel LW 102 mined 



LW 101 



J& 



o°LW 102 o 



2 Panels mined 



LW 101 



Gob o ' 



.o 



•Jfe! 



o°'o 



Gob, 



Figure 11. -Computed stress profiles of multislice trial. 



COST ANALYSIS OF MULTISLICE MINING 



U.S. coal markets are highly competitive. Western thick 
seams are frequently far removed from their market desti- 
nations, and rail costs are high. Thus, multislice mining 
costs should be as low as possible. Using an intermediate 
rock parting for lower slice roof, as planned at the Dutch 
Creek multislice trial, appears to offer good potential to 
keep costs low at this time. As improvements are made in 
multislice technology, other methods, such as using arti- 
ficial roof, may become cost competitive. 

Besides direct costs like labor and material, mine oper- 
ating costs also depend on layout, ground control con- 
ditions, and advance and retreat rates. Different produc- 
tivities are achieved by development and longwall mining, 
which in turn affects the total cost of the coal mined. One 
strategy to reduce cost is to minimize the amount of devel- 
opment work relative to longwall mining by making long- 
wall panels wider. As will be shown later, wider panels 
also increase overall coal recovery. 

A computer model was used to estimate the effect 
layout and ground control conditions might have on multi- 
slice mining using typical western entry systems and an 
intermediate rock parting as a lower slice roof. The ob- 
jectives were to compare relative costs of different devel- 
opment systems and estimate the sensitivity of costs to 
layout geometry, ground control conditions, and in situ 
variables, such as seam and rock parting thickness. A 
determination of selling price, discounted cash flow, or 
rate of return was not intended. 



DESCRIPTION OF COMPUTER MODEL 

The model was developed using Lotus 1-2-3 on an IBM 6 
XT computer. Lotus, in addition to its spreadsheet 
application, allows the user to easily program mine 
relationships. 

DESCRIPTION OF HYPOTHETICAL 
MULTISLICE CASES 

This study analyzes two hypothetical cases of multislice 
mining in order to estimate direct operating costs. The 
cases are derived along the lines of reference (4). The 
associated productivities and resource recoveries are also 
estimated. 

Multislice mining has not been used in the United 
States, so actual geotechnical conditions have not yet been 
experienced. In order to build the cost models, some basic 
assumptions were made. These assumptions are explained 
in detail below. Sensitivity analysis was used to determine 
how sensitive the results are to particular assumptions. 



Reference to specific products does not imply endorsement by the 
U.S. Bureau of Mines. 



18 



PHYSICAL ENVIRONMENT 

The physical environment is common to each of the two 
cases. The underground mine is in the western United 
States and has a level thick coal seam that is split in two 
by a rock parting. The upper and lower splits are each 
10 ft thick, and the parting is also 10 ft thick. 

MINING METHOD AND PLAN 

The upper split is mined first, and, some years later, the 
lower split is mined. The mining method for both the 
upper and lower splits is longwall production with contin- 
uous miner development. In both cases, the upper seam 
longwall is 800 ft wide. Lower seam panels are developed 
within the perimeter of the upper seam panels, offset 60 ft 
inside any upper seam chain or barrier pillars. Two-entry 
development is used (figs. 12-13). Bleeders are driven in 
the upper split, but not in the lower split. Tables 2 and 3 
summarize the cost model assumptions. 

In case 1 (fig. 12), the upper split is developed by one 
3-entry continuous miner section, which develops the gate 
entries, bleeder entries, and starting rooms. The lower 
split is developed by two sections, one from the tailgate 
side and one from the headgate side. Each section drives 
a pair of decline tunnels from the upper split mains to gain 
access to the lower split. Once the lower split is inter- 
sected, the entries are driven, as shown in figure 12. Two 
lower slice development sections are required because both 
the headgate and tailgate entries must be driven for each 
panel. Only one set of tunnels must be driven for each 
panel, however. 

In case 2 (fig. 13), one 2-entry section develops the 
upper split. The lower split is reached, as in case 1, by 
two parallel declines from the upper split mains. After the 
declines intersect the lower split, two gate entries are 
driven so that each is outside the upper seam entries. 
Crosscuts are at 200-ft centers, aligned under the upper 



split crosscuts. Because of the long, widely spaced 
crosscuts, development is slow. Therefore, to keep up with 
the lower split longwall retreat, two panels are always 
being developed simultaneously. 

RESULTS OF ANALYSES 

Table 4 shows the major results for both cases for the 
assumptions in tables 2 and 3. The average unit cost for 
case 2 is 8 pet higher than case l's cost. The case 2 cost 
is higher largely because the lower split longwall must wait 

7 months before development is finished. The slow devel- 
opment results from the widely spaced entries and the 
associated long crosscuts and the fact that only one section 
can be used in such a development configuration. The 
slow development causes the succeeding longwall to wait 

8 months before it can move into the panel. When a 
longwall waits on development, it must pay for labor dur- 
ing the wait time. 

In that case 2's unit cost is higher than that of case 1, 
one would expect case 2's productivity to be lower, and this 
is so. The main reason is the wasted labor caused by the 
waiting of the case 2 lower split longwall on development 
to finish. 

Total combined development and longwall production 
in both cases is within 2 pet. However, keep in mind that 
case 2 develops the upper seam with two entries and case 
1 develops with three entries. If case 2 had developed with 
three entries, both upper seam productions would have 
been identical. Case 2's lost tonnage will be picked up on 
the next panel. 

Case 2 requires about 30 pet longer than case 1 to 
develop the lower split. As mentioned above, the dif- 
ference is directly related to the slow development rate 
assumed for the widely spaced two-entry development 
method. If the longwall did not have to wait this addi- 
tional time, it would mine out the panel in 12.9 months 
instead of 21.6 months. 



Table 2.-Cost assumptions common to case 1 and case 2 



Mining height, ft: 

Coal-longwall 

Development 

Interburden thickness ft . 

Specific gravity, lb/ft 3 : 

Coal 

Rock 

Workdays per year 

Average shearer tram speed, 

cycle ft/min . 

Average turnaround time min . 

Rock tunnel decline pet . 



10 

8 

10 

82 
150 
225 

20 

7 
10 



Operating shifts per day: 

Development 

Longwall 

Annual salary costs: 

Laborer 

Salaried , 

Fringe pet of annual wage 

Material and maintenance cost per 

short ton: 1 

Development , 

Longwall 

Rock tunnel 



3 
2 

$27,000 

$33,000 

40 



$6 

$4 

$12 



includes power, supplies, and parts; no labor is included. Development and longwall estimates were derived from an operating longwall 
mine. Rock tunnel costs were estimated at twice those of development work. 



19 



Table 3.-Separate cost assumptions for case 1 and case 2 

Case 1 Case 2 

Upper Split Lower Split 1 Upper Split Lower Split 1 

Longwall face length . . . ft . . 800 584 800 602 

Outby barrier ft . . 400 460 400 460 

Inby barrier ft . . 400 NAp 400 NAp 

Development entries 3 2 2 2 

Entry centers ft . . 60 48 60 180 

Entry width ft.. 18 18 18 18 

Crosscut centers ft . . 100 100 100 200 

Starting rooms 2 2. 2 2 

Crew sizes: 
Development, per shift: 

Laborer 8 7 7 7 

Salaried 1 1 1 1 

Longwall, per shift: 

Laborer 9 8 9 8 

Salaried 1 1 1 1 

Special crews-per day: 

Laborer 8 8 8 12 

Salaried 2 2 2 3 

Material difficulty factor: 2 

Development 1.0 1.3 1.0 1.5 

Longwall 1.0 1.2 1.0 1.2 

Panel advance, ft per month 
per shift: 

Development 3 160 176 176 50 

Longwall 4 150 195 150 190 

NAp Not applicable. 

x Lower split face length is determined from the geometry of the upper and lower splits. 

2 Used to increase material and maintenance costs per short ton because of poorer geotechnical conditions in the lower split. Material and 
maintenance costs are multiplied by this factor to obtain lower split costs. 

3 Basic panel advance rate for a 3-entry system was set at 160 ft per month per shift. This rate is increased by 10 pet when 2-entry 
development is used. Rate for case 2 in lower split was estimated taking into account the long crosscuts that may exacerbate ventilation 
problems and which will increase average tramming times. 

4 Upper seam retreat rate was assumed at 150 ft per month per shift. Lower seam face length is shorter because it is set in from the upper 
split entries. At the same tram speed and turnaround time end time, lower split retreat rate is mathematically faster than upper split longwall. 



Table 4-Cost analysis results for case 1 and case 2 



Case 1 



Case 2 



Case 1 



Case 2 



Cost per short ton: 

Average 1 

Upper split 

Lower split 

Productivity, st/employee-shift: 

Average 

Upper split 

Lower split 

Production, st: 

Total 

Upper split 

Lower split 



7.72 
6.78 
8.77 

58.94 
68.17 
49.72 

3,135,450 
1,813,081 
1 ,322,369 



8.12 

6.64 

10.48 

48.41 
73.16 
33.17 

3,060,750 
1 ,762,330 
1,298,419 



Time, months: 
Upper split: 

Development and access . . 17.8 

Longwall 18.8 

Lower Split: 

Development and access . . 14.3 

Longwall 15.1 

Resource Recovery, pet: 

Total 68.0 

Upper split 78.6 

Lower split 57.3 

Upper split portion of total . . . 39.3 



16.3 
18.8 

20.8 
21.6 

70.9 
81.6 
60.1 
40.8 



Average, weighted by tonnage, of the upper and the lower split costs. 



20 



Bleeders 



DD 



Upper split Inby barrier pillar 



Upper split longwall starting rooms i 



400' 




M J /'. ,, ." T •"■ • ■•• — • • ■ "■ *.■ . i"*'''< • . » » .1 ■ . — r-i — > — 7 . . « — r: — .. » i L ' 



■^li, {.. •••,•• ■ ■ • ■/ ••• ■ •'-, i-i . V j " " ■ ••! v .. ; -j v . '-- . • ■ ,, <■ ••••'•■ ■■; 

« « |f**l ■.! ■ J " . » . ' " ' ■ ' I'lll '' ' A.I... 1 . 1 . *.'** . ■ I. . •—*• *» . ■ 1** . .1 .I.U Tj,' 

.«..•..'.■*. . .• ^ »■■■■. . t ^ 1 _ I | .. . .,. «..»..«-.. i. . .i '**; rif' 



Lower split longwall starting rooms 



DD 




HDD 
HDD 
HDD 



Mains 



Figure 12.-Case 1 multislice development layout. 



21 



Bleeders 



D 



D 



Upper split lnby barrier pillar 



Upper split longwall starting rooms | 



400' 




HDD 



1 II II 1 


1 II II 1 


1 II II 1 



Mains 



Figure 13.-Case 2 multislice development layout. 



22 



Sensitivity Analysis 

Many factors can affect operating costs. The model 
examined the cost effects of multislice layout dimensions, 
development entry type, development and retreat rates, 
and the difficulty of mining. Each of these factors was 
varied in the model to determine the sensitivity of costs to 
the factor. 

Cost is the direct operating cost and includes both 
development and longwall costs. Six cost sensitivities were 
run. 

Upper Split Face Length 

Figure 14 shows that, as the upper split face length 
decreases, lower split costs rise faster than upper split 
costs. The lower split longwall face length is a function of 
the development geometry, the inset, and the upper split 
longwall face length. As the upper split face length de- 
creases, the lower split face length decreases foot for foot. 
Percentagewise, the lower split face shrinks faster. As face 
length shrinks, gate entry development stays constant; the 
only development saved is the starting rooms are shorter 
by the exact amount of the face shrinkage. Therefore, the 
time required for the longwall to mine a panel shrinks 
faster than the time to develop a panel; hence, as the 
upper seam face reduces, the lower seam longwall must 
wait longer on development. 

Number of Upper Split Development Entries 

Figure 15 shows that as the number of upper split de- 
velopment entries increases, costs rise. Upper split costs 
rise because each additional entry requires more labor and 
material and, most importantly, more time. Hence, the 
upper split longwall is more likely to wait longer on de- 
velopment as the number of entries increases. Lower split 
costs are unaffected. 

Upper Split Longwall Retreat Rate 

Figure 16 shows that the cost benefit of a faster upper 
split longwall retreat rate has a limit. The cost improve- 
ment levels out quickly at the point where the retreat rate 
is so fast that development cannot keep up. As soon as 
the longwall must wait on development, there is no benefit 
to having a speedier longwall. 

Material-Maintenance Factors 

Figure 17 shows the effect of the material-maintenance 
factors, which are used to adjust development and longwall 
costs for difficult conditions in the lower split relative to 
the upper split. For example, if the user believes the lower 
split development and longwall will experience poorer 
ground conditions than in the upper seam, requiring more 
roof bolts, the user can insert a factor by which lower split 
material-maintenance costs are increased from those in 
the upper split. Longwall factors increase total unit costs 



faster than development factors because material- 
maintenance costs are a higher percentage in the longwall 
costs than in development costs. 

Interburden Thickness 

Figure 18 shows that interburden thickness plays a 
small role in the cost of the model. The only effect that 
interburden has in the model is its effect on the access 
tunnel length between the upper and lower splits. 

Case 2 Lower Split Development Rate 

This case was run because the assumed development 
rate was so slow (50 ft per month per shift, table 3). Fig- 
ure 19 shows that should development speed increase, the 
total cost would decrease. 



KEY 
□ Upper split 
O Lower split 
A Overall 




500 



600 



700 



800 



UPPER SLICE LONGWALL FACE LENGTH, ft 

Figure 14.-Costs versus upper slice face length, case 1 
(upper) and case 2 (lower). 



23 




r 



2 3 4 
NUMBER OF DEVELOPMENT ENTRIES 



KEY 
Lower spl i t 

Upper spl i t 
Overal I 



Figure 15.-Cost versus number development entries for case 1. 



CD 

s 

m 




^.50 



120 



140 



160 



180 



200 



220 



RATE I per month, per shift), ft 



Figure 16. -Cost versus long wall retreat rate, case 1 (upper) 
and case 2 (lower). 





8 


60 




8 


40 




8 


20 




8 


00 




7 


80 




7 


60 




7 


40 


I- 
co 


7 


20 



— 


I I 


I I I I 

KEY 


I I , 




□ 


Development s* 




— 


O 


Longvall Q; 




— > 


I I 


I I I I 


I I 




7.50 



0.80 1. 10 1 . 40 

MATERIAL-MAINTENANCE FACTORS 



1.70 



$7.78 



$7.76 — 



$7. 74 



CD 

m 






O 

CJ 


$8 


72 
l" 7 


< 






o 


$8 


16 




$8 


15 




$8 


14 




$8 


13 



58. 12 




10 



15 20 

INTERBURDEN THICKNESS, ft 



Figure 17. -Cost versus material-maintenance factor, case 1 
(upper) and case 2 (lower). 



Figure 1 8.-Cost versus thickness of intermediate rock parting, 
case 1 (upper) and case 2 (lower). 



24 



KEY 

o Lower split 
□ Overall 




25 50 75 100 
RATE (per month, per shift, per section) ft 

Figure 19.-Cost versus lower slice development rate for case 2. 



KEY 



Productivity 

Productivity is closely related to the inverse of costs. 
Figure 20 shows the relationship between the upper seam 
face length and overall productivity. As face length in- 
creases, productivity increases. The change in slope be- 
tween 600 and 700 ft is caused because the model ad- 
ded operating and maintenance people to a face when its 
length exceeded 650 ft, causing a step function to occur. 

Resources Recovery 

Mining of the lower slice in case 1 increased the coal 
recovered from 39.3 pet (upper slice mining only) to 68 pet 
of the total, using an 800-ft-wide upper slice panel. In 
case 2, recovery was increased from 40.8 to 70.9 pet. The 
case 2 recovery is slightly higher because fewer chain pil- 
lars are left behind. Figure 21 shows that total recovery 
from both upper slice and lower slice mining increases as 
much as 10 pet when the upper slice panel is widened from 
500 to 800 ft. 



Overall 



r 

CO 

I 

0j 

> 

o 

Q. 
E 
CO 



A Lower split 
O Upper split 
T 




500 600 700 800 

UPPER SLICE LONGWALL FACE LENGTH, ft 

Figure 20.-Productivity versus upper slice face length, case 1 
(upper) and case 2 (lower). 



■H 

O 

a 



>- 
cr 

> 
o 
(J 
u 
cr 

UJ 

o 
cr 

D 
O 
CO 
UJ 

cr 




500 600 700 800 

UPPER SLICE LONGWALL FACE LENGTH, ft 

Figure 21. -Resource recovery versus upper slice face length, 
case 1 (upper) and case 2 (lower). 



25 



SUMMARY AND CONCLUSIONS 



Multislice methods of mining coal are not now used in 
the United States, but are used, or have been used, in at 
least 13 countries throughout the world. These methods 
were researched to determine their layout and conditions 
under which they are applied. Ascending multislice is used 
in thick, pitching seams, usually with stowing material 
(backfill) to support the undercut roof. Descending meth- 
ods are used in flatter seams. As many as 10 consecutive 
underlying slices have been mined in China. Longwali is 
most commonly used, but a room-and-pillar multislice 
method has been tested in Australia. 

The geologic, ground control, and cost problems of 
applying multislice mining in the United States were 
analyzed, and the method appears to be feasible for ex- 
tracting thick western coal seams. Standard longwali 
mining techniques can probably be used. U.S. safety, eco- 
nomic, and legal requirements will dictate the actual use- 
fulness of the method. Safety requirements relating to 
ground control were discussed in this report. Spontaneous 
combustion is an additional safety problem that must be 
addressed if susceptible seams are mined. Highly com- 
petitive American coal markets require that lower slices 
have operating costs within the cost range of standard 
longwali mining. To satisfy U. S. legal requirements, stan- 
dard American longwali development practice is needed. 
Further conclusions are presented under the headings of 
multislice methods, ground control, geology, and cost. 

MULTISLICE MINING METHODS 

The multislice method, believed by the authors to be 
best adapted to western thick seams, is nonsimultaneous 
descending longwali without the use of artificial roof or 
backfill material. Other methods, such as ascending multi- 
slice with backfill, may have application to thick, steeply 
dipping western seams. The easiest condition in which to 
initially use multislice mining may be a thick seam con- 
taining a substantial rock parting that can be used as a 
lower slice roof. 

The use of artificial roof (wire mesh or other material) 
is considered currently too expensive to compete in de- 
manding U.S. coal markets. Similarly, simultaneous multi- 
slice and the use of stowing or backfill material are also 
considered expensive. Simultaneous multislice productivity 
can be reduced by the requirement to keep a constant 
distance between faces, which may require one face to wait 
on the other during breakdowns. Improvements in multi- 
slice technology, such as mechanized artificial roof instal- 
lation may, in the future, make some methods more cost 
competitive. 

GROUND CONTROL AND SPONTANEOUS 
COMBUSTION 

Ground control in lower slices is a major safety con- 
cern. If a roof fall occurs in lower slice development en- 
tries or longwali faces, it might propagate into the upper 



slice gob, allowing rubblized gob material to fall into lower 
slice workings. To prevent this occurrence, some method 
is needed to separate the upper and lower slices. Artificial 
roof material is frequently used for this purpose, but re- 
quires extra expense, and its installation may slow the 
longwali face, reducing productivity. Some western seams 
contain rock partings that now prevent recovery of the 
section of the seam above or below. The parting, if suffi- 
ciently competent, might be used to separate the slices and 
provide a lower slice roof. Another means to reduce the 
risk of a lower slice roof fall propagating into upper slice 
gob is to allow the upper slice gob to consolidate. If the 
rock split is to be used as lower slice roof, its competency 
should be thoroughly investigated prior to mining. 

For purposes of planning and ground control analysis, 
multislice mining can be divided into four stages: (1) ac- 
cess, (2) development, (3) longwali mining, and (4) face 
recovery. Successful completion of each of these stages is 
necessary to successful multislice mining. During the 
access stage, development entries must be driven to the 
lower slice. If a rock-split lower slice roof is used, then 
the entries must penetrate through the split. Zones of 
high abutment stresses on the panel edges and ends should 
be avoided or roof support increased in these areas. De- 
velopment entries should also avoid abutment stress zones 
and should be designed to provide maximum protection 
against roof fall. Longwali faces should be situated to 
obtain best possible ground control conditions. During 
recovery of the face, the integrity of the rock split used for 
lower slice roof should be ensured. 

Ground control observations during multiple seam 
mining and measurements of gob pressure indicate that a 
zone of decreased vertical stress exists beneath upper slice 
gob. This destressed zone is generally a favorable location 
for lower slice workings. A corresponding high stress zone 
exists on panel edges, and it should be avoided. Com- 
monly, multislice layouts have lower slice workings situated 
entirely beneath upper slice gob. The lower slice entries 
are inset horizontally from upper slice entries to avoid 
abutment stresses. 

A joint industry-Bureau test of multislice mining is 
planned at a deep mine in western Colorado. The lower 
slice layout incorporates head-tail entries inset 60 ft inside 
upper slice entries and two-entry development. Standard 
three-entry development was not selected because it has 
four-way intersections that might decrease roof stability. 
Retreat longwali is planned to permit probing lower slice 
ground control conditions prior to committing longwali 
equipment. 

Spontaneous combustion may be a serious problem 
if multislice mining is used in susceptible western thick 
seams. Leakage of spontaneous combustion gaseous prod- 
ucts, such as carbon monoxide, from upper slice gob into 
lower slice workings would be a serious hazard to miners. 
Separating upper and lower slices becomes important from 
both ground control and spontaneous combustion perspec- 
tives. In the case of spontaneous combustion, a seal 



26 



between the slices becomes highly desirable. Measures 
that have been taken to provide the seal include allowing 
time for gob consolidation, injecting water to accelerate 
the consolidation, and injecting cementitious material, 
washery waste, or loess mud to solidify the lower part of 
the upper slice gob. 

GEOLOGY 

The geology of the coal seam, roof, and floor is impor- 
tant in multislice mining as in standard longwall. The 
geologic factors that are especially important in multislice 
mining are (1) coal thickness, (2) the competency of the 
rock split separating the slices, and (3) factors affecting 
gob consolidation. A decrease in seam thickness caused by 
geologic anomalies or thinning could reduce thickness 
below equipment minimum heights. The competency of 
the rock split separating the slices depends on its thickness, 
bedding planes, and lithology. These features are deter- 
mined by the original depositional environment of the 
thick seam. Other features, such as joints and fractures, 
also affect the competency of the rock split. The time 
required for upper slice gob to consolidate and the con- 
solidation reached depends on the gob material, its sus- 
ceptibility to water, the amount of water present, and 
overburden pressure. Years may be required for some 
gobs to consolidate. 

COST SENSITIVITY ANALYSIS 

An analysis was done to determine the sensitivity of 
combined development and longwall operating costs to the 



type of lower slice development, layout dimensions, and 
other variables. Two cases were examined. The first case 
requires separate head-tail entries for each lower slice 
longwall and in the second case, adjacent lower slices 
share a single development system. In both cases, wider 
upper slice faces (and correspondingly wider lower slices) 
increased productivity and lowered cost. Case 2 had 
higher costs because the long tram distances required for 
the development continuous miner reduced development 
rates and required the longwall to remain idle until de- 
velopment caught up. The cost of accessing the lower slice 
through the intermediate rock split does not appear to 
greatly increase cost. 

REMAINING PROBLEMS 

Although multislice mining has not yet been used in the 
United States, its use appears technically and economically 
feasible in thick western coal seams. A number of ques- 
tions need to be answered before the method gains broad 
acceptance by the mining industry. They include actual 
costs and productivities, best development system and 
placement of lower slice entries, and lower slice roof sup- 
port. Scheduling of lower slice development may be a 
problem if that development cannot keep pace with long- 
wall faces. Spontaneous combustion may be a safety prob- 
lem in susceptible seams. Ventilation is an important 
consideration, but was not analyzed in this report. It is 
possible that the answers to these questions will be mine 
specific, and actual operating experience will be needed to 
get the answers. 



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•U.S. Government Printing Office: 1990— 511-010/40000 



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